Dentists, orthodontists, periodontists, and oral surgeons typically use electromagnetic radiation (e.g., x-rays) to obtain images of a patient's teeth, mouths and gums to aid in diagnosis and treatment. In traditional oral and dental radiography, a photographic film is placed in the patient's mouth, for example behind a patient's tooth, and an x-ray beam is projected through the tooth and onto the film. The film, after being exposed, is developed in a dark room or a closed developer using special chemicals to obtain a photographic image of the tooth.
Recently, the field of filmless dental radiography has emerged. In filmless dental radiography, an x-ray beam is still projected through the patient's tooth, but no photographic film is used. Instead, an electronic sensor is placed in the patient's mouth behind the tooth to be examined. The electronic sensor may include a charge-coupled device (CCD), a complementary metal-oxide semiconductor (CMOS) active pixel sensor (APS) array or any other filmless radiation sensor. The x-rays pass through the tooth and impinge on the electronic sensor, which converts the x-rays into an electrical signal. The electrical signal is then transmitted to a computer to produce an image on an associated output device, such as a monitor or a printer.
Minimizing the patient's exposure to x-rays and obtaining an accurate image are of concern when using a filmless dental radiography system. These systems typically utilize an x-ray source and an intraoral sensor. In these systems, it is often desirable to maximize the sensitivity of the sensor so that a short x-ray pulse can be used to minimize the patient's exposure time. However, the x-ray source and the sensor are often sold as separate components with, typically, no communicative link between them. Thus, the radiography system cannot tell when to begin the image acquisition process (i.e., the sensor cannot tell when x-rays are being emitted from the source).
One approach to this problem has been for the radiography system to acquire a partial frame of an image that is output by the intraoral sensor. The system then digitally analyzes the partial frame and checks the average grey level of the image. If the grey level changes beyond a set threshold the system can assume that x-rays are present and it can begin a full frame capture. The drawback of this approach is that it takes additional time and uses a significant portion of the x-ray pulse (e.g., 10-20%), thereby increasing the time a patient is exposed to radiation. Since a significant portion of the x-ray pulse is used to detect the presence of the radiation, there is less time available to acquire the actual image. Therefore, the patient's radiation dose is often increased.
Another approach to this problem has been to add diodes in the corners of the intraoral sensor to detect the presence of x-rays. However, this approach could prove inadequate because the diodes do not cover the entire field of view, resulting in missed x-ray pulses. The more missed pulses, the higher the patient's dosage of radiation is needed before the x-rays are detected and an image is acquired.